Double effect evaporators and method of operating same



March 22, 1960 J. BEAGLE, JR, EI'AL 2,929,443

DOUBLE EFFECT EVAPORATORS AND METHOD OF OPERATING SAME Filed June 4, 1956 .57

Z Par/re?" Carier' I fltzorz? eys Unite rates DOUBLE EFFECT EVAPORATGRS AND METHBD F OPERATING SAME Application June 4, 1956, Serial No. 589,084

13 Claims. (,Cl. 159-20) This invention refers to doube etfect evaporator units or the like, and relates more particularly to improved evaporator structure and method of operating it.

A primary object of this invention is the provision of a double effect evaporator unit for use in a rayon spin bath system or the like, designed to effect substantial steam economy as compared to conventional rayon open bath evaporation.

Another object is a double effect evaporator unit of the above type designed to reduce the heating load on the first effect.

Another object is a double eifect evaporator unit designed for improved steam economy at high feed to evaporation ratios.

Another object is a double effect evaporator unit and a method of operating it to eliminate the normally present liquor flash tank.

Another object is a double effect evaporator unit designed so that the amount of liquor fed to the first effect from the second effect discharge is set as low as possible while still preventing salting out of the crystals in the tubes of the first effect.

Another objectis a double effect evaporator unit of the above type designed and arranged with the feed to the first effect regulated so that the discharge from the first effect is justbelow saturation.

Other objects will appear from time to time in the ensuing specification and drawings in which:

Figure l. is a diagrammatic layout of one form of this invention;

Figure 2 is a variant form; and

Figure 3 is a further variant.

in Figure 1, the first effect evaporator, with regard to vapor flow, has been indicated generally at it), and it will berecognized as a conventional long tube vertical evaporator having a vaporhead 14 and lower shell 16. The shell 16 has a tube bundle which is supplied with steam through a suitable inlet at 18 for the first effect and a condensate discharge 2%. The vapor outlet pipe 22 from the top of the first effect is led through a suitable pipe to the steam inlet 24 for the second effect 12, and a suitable condensate discharge 26 is provided to dispose of the condensate. The vapor pipe 28 from the second effect is led to a suitable condenser 3% in the usual manner.

An inlet line 32 from a rayon spin bath process or the like, brings what is referred to as spin bath or dilute feed to a liquor inlet 34 on the second effect 12. The concentrate discharge from the second effect leaves by the usual liquor outlet 36 and through a suitable pipe 355 is carried back. as a concentrate solution to be used in the rayon spin bath process or the like.

A small predetermined amount or quantity of the concentrate discharge from the second effect is fed back by a suitable pipe 4% with a suitable valve 41 to the liquor inlet 42 for the first effect. The amount returned will be described later. Suflice itto say for the moment that atent ine it is small compared to the total amount of circulating spin bath. Additional vapor is removed from this allines 38 and 5% have been shown as separate, these could be combined at any suitable point in Figure 1. Vapor from the flash tank may be taken to the condenser 30 by a suitable line 51. v

The amount of liquor fed to the first effect, by way of conduit 40, from the second effect discharge, which has been characterized as small, is actually set'as low as possible while at the same time preventing salting out of the crystals in the first effect tubes. This is to say that the feed is regulated so that the discharge from the first effect is just below the saturation point.

A variant has been shown in Figure 2 and parts or elements the same as in Figure 1 have been designated by the same numerals. The Figure 2 form differs fromthe Figure 1 system in that the liquor discharge from the first effect, indicated at 52, which corresponds to the discharge 44 in Figure 1, instead of being cooled in a flash tank, as at 48 in Figure l, is returned by a suitable pipe 54 and cooled by mixing with the dilute feed 32 to the second effect. There is no objection to mixing the concentrate discharge from the first effect with the dilute feed to the second effect.

Control or regulationof. the small amount of liquid,

feed to the first effect by way of conduit 40 from the second effect might be done manually. The operator could observe the discharge from the first effect and regulate valve 41. The operator could take samples of the discharge from the second effect, at line 46 in Figure 1 or at line 52 in Figure 2, to determine if crystallization was present. He could reduce the flow in line 4h until crystals appeared in the second effect discharge. Then he could increase the flow slightly by manually adjusting the valve 41. This is more or less a trial and error method and is common in practice.

A further variant has been shown in Figure 3. A well recognized problem with double effect evaporator systems when concentrating rayon spin bath solutions is the tendency of sulphur compounds carried in the vapor from the first effect to collect on the outside of the tube in the bundle of the second effect. This scale or deposit is extremely difficult to remove and can result in a lot of down time. In Figure 3, the vapor pipe 56 from the first effect has been connected to the steam inlet header of a horizontal steam tube evaporator designated generally at 58. This horizontal tube evaporator serves as the second effect unit and is supplied with the dilute feed from the system at 60. The vapor carrying the sulphur compound from the first effect will form a scale ordeposit on the inside of the tubes in the second effect evaporator. But this scale can be much more easily removed from the inside of the tubes than from the outside by mechanical means, for example, brushes and the like, or chemical means, such as, various suitable flushing compounds. It should be understood that a small amount of the concentrate discharge from the second effect may be carried by the line 40, as in the prior embodiments, to the liquor inlet for the first effect, and the concentrate discharge from the first effect may be carried either to a separate flash tank, as indicated by the line 62, or directly to and mixed with the dilute feed to the second effect inlet, as by the line 64.

The use, operation and function of this invention are as follows: I

An important consideration in double effect evaporator systems is steam economy, which can be a substantial factor in a rayon spin bath process. It is desirable to degas all of the circulating spin bath, or as much as possible, so that a considerable quantity of the sulphur compounds will be removed as gas. This results in high feed to evaporation ratio units, which is the ratio of the Weight of liquid fed to the weight of water evaporated. For example, this ratio could be 50-100 to 1, although it is by no means limited as a ratio of -1 should be considered.

The disclosed invention substantially reduces the heat load in the first effect evaporator and improves the steam economy. The amount of liquor fed to the first etfect from the second effect discharge, which has been characterized as small, may be set as low as possible, but at the same time preventing salting out of crystals in the first 'ezfect tubes. The feed can be easily regulated so that the discharge from the first effect evaporator is just'below the saturation point. The discharge from the first elfect evaporator can be passed through a flash tank to further concentrate and cool the liquor, as in Figure l, or it may be combined directly with the spin bath feed as there is no objection to mixing the concentrated discharge from the first effect with the dilute feed to the second effect. The arrangement has the additional advantage that as the feed to evaporation ratio increases, degasification will be more effective in the second eifect and'there will be less sulphur compounds evolved in the first effect. This substantially lengthens the boilout cycle for the heater tubes in the second effect and reduces down time. An additional advantage is areduction in the water consumption in the condenser in addition to the higher steam economy due to the limiting of the liquor flow to the first efifect evaporator to a minimum. The heat transfer between the liquor and vapor is better with the liquor on the inside of the tube as in Figures 1 and 2. However, if the vapor is passed through the tubes in the second elfect as in Figure 3, the scale can be more easily cleaned out by a brush or the like. One of the important points is that the vapor from the small stream in the first effect evaporator has less sulphur compounds, so the deposit problem in or on the tubes on the second elfect evaporator is substantially reduced. Such an arrangement as in" any one of the disclosed embodiments will effectively and efliciently balance a spin bath system by removing water from the bath, and substantially all of the bath flowing through the evaporator will be degassed.

Also, the control of temperature of the spin bath return to the system is a matter of major concern to an operator. The concentrate discharge from the first elfect, at 54 in the Figure 2 form, will be cooled by mixing with the cooler dilute feed in line 32 to the second elfect. In the Figure 1 form, cooling of the concentrate discharge from the first effect is by fiashing in tank 48. In the Figure 3 form, it could be either way.

The point is that the evaporator discharge returned to the spin bath system should be at a relatively constant temperature, and cooling in the above manner is important in maintaining the evaporator discharge tempera ture constant.

Whereas several embodiments of the invention have been shown and described, various other modifications,

substitutions, alterations and changes are possible. It is,

therefore, desired that the invention be unrestricted except as by the appended claims.

We claim:

1. In a double effect evaporator unit for use in a circulating spin bath system or the like, first and second effect evaporators, each having a liquor inlet and outlet, a vapor inlet and condensate outlet, and a vapor outlet, the liquor inlet on the second eifect evaporator being constructed to receive dilute feed liquor from the system,

the liquor outlet on the second effect evaporator being constructed to return the concentrate discharge liquor to the circulating spin bath system, a connection between the liquor outlet on the second effect evaporator and the liquor inlet on the first effect evaporator supplying only a fractional part of the concentrate discharge liquor from the second effect evaporator to the first effect evaporator, a connection between the vapor outlet on the first effect evaporator and the vapor inlet on the second effect evaporator, and means forcooling the liquor discharge from the first effect evaporator so that the small amount of concentrate discharge liquor from the first etfect evap' orator will be cooled at least no later than when it is returned to the circulating spin bath system.

2. The structure of claim 1 in which said cooling means includes a connection from the liquor outlet on the first efiect evaporator to the liquor inlet on the second efiect evaporator so that the small amount of concentrate discharge liquor from the first effect evaporator will be mixed with and cooled by the dilute feed liquor from the system.

3. The structure of claim 1 in which the connection from the liquor outlet on the first effect evaporator is connected to a flash tank, the concentrate discharge liquor outlet from. the flash tank being connected to the return line to the circulating spin bath system.

4. The structure of claim 1 in which both evaporators are the long tube vertical type each having a lower shell and vapor head. 1

5. The structure of claim 1 in which the first effect evaporator is the long tube vertical type with a lower shell and vapor head, and the second effect evaporator is the horizontal tube type.

6. The structure of claim 1 in which essentially all of the dilute feed liquor from the system is fed to the second etfect evaporator.

7. A method of operating-a double effect evaporator unit with a rayon spin bath system or the like, the unit having first and second efiect evaporators, each with a liquor inlet and outlet, a vapor inlet and condensate outlet, and 'a vapor outlet pipe, including the stepsof supplying dilute feed liquor from the rayon spin bath system to the liquor inlet of the second effect evaporator, returning all but a small portion of the concentrate discharge liquor from the liquor outlet on the second effect evaporator to the circulating spin bath system, supplying the small portion of the concentrate discharge liquor from the second effect evaporator to the liquor inlet on the first effect evaporator, supplying the vapor from the vapor outlet pipe on the first effect evaporator to the vapor inlet on the second effect evaporator, and cooling the small amount of concentrate discharge liquor from the liquor outlet on the first elfect evaporator and returning it to the circulating spin bath system. i

8. The method of claim 7 characterized in that the step of cooling the small amount of concentrate discharge liquor from the liquor outlet on the first effect evaporator to the system includes returning it to the inlet line from the system so that the concentrate discharge liquor from the first effect evaporator will be mixed with the dilute feed liquor to cool said concentrate discharge liquor and be returned to the liquor inlet on the second effect evaporator.

9. The method of claim 7 characterized in that the cooling step includes the step of supplying the small amount of concentrate discharge liquor from the liquor outlet on the firsteifect evaporator to a flash tank, and returning the concentrate discharge liquor from the flash tank to the circulating spin bath system.

10. The method of claim 7 characterized by the step of regulating the quantity of the small amount of liquor supplied from the second effect to the first eifect evaporator so that the discharge liquor from the first effect is just slow the saturation point.

11. The method of claim 7 characterized by supplying aeaaets 7 essentially all of the dilute feed liquor from the system tothe liquor inlet of the second efiect evaporator.

12. A method of operating a double efiect evaporator unit with a rayon spin bath system or the like, the unit having first and second elfect evaporators, each with a liquor inlet and outlet, a vapor inlet and condensate outlet, and a vapor outlet pipe, including the steps of supplying dilute feed liquor from the rayon spin bath system to the liquor inlet of the second elfect evaporator, returning all but a small portion of the concentrate discharge liquor from the liquor outlet on the second efiect evaporator to the circulating spin bath system, supplying the small portion of the concentrate discharge liquor from the second efiect evaporator to the liquor inlet on the first effect evaporator, supplying the vapor from the vapor outlet pipe on the first eflect evaporator to the vapor inlet on the second effect evaporator, and cooling the concentrate discharge liquor from the liquor outlet on the first effect evaporator.

13. The method of claim 12 characterized in that the step of cooling the concentrate discharge from the liquor outlet on the first effect evaporator includes feeding it to the liquor inlet of the second eflect evaporator so that it will be cooled in the second efiect evaporator.

References Cited in the file of this patent UNITED STATES PATENTS 2,262,519 Talton Nov. 11, 1941 2,631,926 Eckstrom Mar. 17, 1953 2,744,571 Mair May 8, 1956 2,747,382 Sloan May 29, 1956 2,764,234 Rauh Sept. 25, 1956 FOREIGN PATENTS France Mar. 31, 1943 

